1. Field of the Invention
This invention relates generally to reclosing relays, and more specifically to a reclosing relay employing a redundant lockout scheme to prevent circuit breaker pumping.
2. Description of the Prior Art
Electrical transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits that can cause collapse of the power system and serious and expensive apparatus damage. For instance, such a fault condition is caused by lightning-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a fault condition, line currents can increase to several times the normal value, thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line are isolated from the network by protective relays. The protective relays, by continuously monitoring ac voltages and currents, locate line faults and initiate line isolation via tripping of the appropriate circuit breakers. These faults are located by examining the relationship of the ac voltages and currents.
The large majority of faults on transmission line conductors are transient and can be cleared by momentarily deenergizing the transmission line. Utility reports indicate that less than 10% of all faults are permanent. It is therefore feasible to improve continuity of electrical service by automatically reclosing the circuit breaker after the protective relay opens it. This is the function of a reclosing relay. The reclosure should be timed to allow deionization of the arc. Automatic reclosing greatly improves service in radial distribution transmission line systems where continuity of service is directly affected by circuit interruption. High speed reclosing on tie lines, if successful, also assists in maintaining system stability.
One type of prior art reclosing relay monitors the status of the circuit breaker and provides a single reclose command to the circuit breaker when it opens. Upon reclosure, the reclose circuit is locked out. If the breaker reclosure is successful, the reclosing relay returns to its normal state after a reset timer completes its timing cycle. This type of reclosing relay is a single-shot reclosing relay.
Another type of prior art reclosing relay offers two attempts at reclosing the circuit breaker. Following circuit breaker tripping, a high-speed unit in this reclosing relay recloses the breaker after a predetermined time has elapsed and thereafter is locked out. Coincident with the high-speed reclosing command, the high-speed unit activates a time-delay unit. After the time-delay period, the time-delay then recloses the circuit breaker and locks out; it must be manually reset. If the circuit breaker remains closed after a reclosing command from either the high-speed unit or the time-delay unit, the reclosing relay is reset after the timer associated with that unit reaches the end of its cycle.
The lockout feature of the prior art protective relays prevents pumping of the circuit breaker. This condition occurs when there is a permanent fault on the transmission line and the reclosing relay causes the circuit breaker to reclose into that permanent fault. The circuit breaker then immediately opens. Without the lockout function, the reclsoing relay recloses the circuit breaker again, and since the fault is still extant, the protective relay monitoring the transmission line retrips to open the circuit breaker. This continuous opening and closing of the circuit breaker, which is called pumping, can cause circuit breaker and system damage. Lockout after reclosure is designed to prevent pumping.
In practice, it has been discovered that this prior art lockout scheme wherein lockout occurs when the circuit breaker recloses, does not provide a sufficiently reliable anti-pumping feature. It is desirable to provide a redundant lockout mechanism to ensure lockout of the reclosing relay if component failure causes a prior-art type of lockout mechanism to fail. It is also desirable for this redundant lockout system to be activated by a reclosure followed within a predetermined interval by a retrip.